1. Field of the Invention
The invention relates to the preparation of alkali metal silicates, and especially to sodium silicates, as these are used commercially for a wide range of applications.
2. Description of Related Art
Most sodium silicate liquors are produced by the Furnace route in which soda ash (sodium carbonate) and sand are fused together at high temperatures, e.g. around 1500.degree. C. and the resulting glass is dissolved under pressure . Silicates in a wide range of grades can be produced in this manner, varying from those having Si0.sub.2 :Na.sub.2 0 weight ratio in the range 1.65 to 2.8 (often referred to as alkaline grades) to grades having higher ratios, e.g. up to about 4.0 (often referred to as neutral grades even though their solutions generally have pH values in excess of 11). Unfortunately, this route is highly energy dependent, and hence expensive to operate.
An alternative, lower energy, process, known as the Hydrothermal process, is also currently being operated. This process involves digestion of sand with caustic soda under pressure, and with much lower temperatures being required, significant savings in energy and manpower costs are achievable by this route. Unfortunately this process is unable to produce neutral grades. It has previously been proposed to overcome this by electrolysis of the resulting sodium silicate liquor in a standard electrolytic cell, having an anode compartment (containing an anode) and a cathode compartment (containing a cathode), the two compartments being separated by a single cation exchange membrane to keep the electrolysis products apart. This relies on an anode reaction to destroy hydroxyl ions in the anode compartment (forming water and releasing gaseous oxygen) while forming new hydroxyl ions in the cathode compartment (with release of hydrogen at the electrode). The driving current is carried through the membrane by the sodium ions passing through it and the net effect is the removal of sodium hydroxide from one stream to the other, even though the individual hydroxyl ions lost from one stream are not those being newly created in the other. Such electrolytic methods for increasing the SiO.sub.2 :Na.sub.2 O weight ratios are claimed to be effective for dilute solutions, i.e. water contents of at least 75 weight %, and hence for enabling neutral grades to be produced via the Hydrothermal process more cheaply than via the Furnace route.